1. Field of Invention
The invention relates to a hydraulic servo device for actuating frictional engagement elements to shift an automatic transmission.
2. Description of Related Art
In an automatic transmission, a band brake is fastened on a drum and is used as a frictional engagement element. The band brake is connected to a specific reaction element, and retains the reaction element on the transmission case so that gear stages may be changed. In case of the band brake, a hydraulic servo device is usually used as an actuator in fastening the band on the drum. This hydraulic servo device includes a cylinder, a piston fit to slide within the cylinder, a rod for outputting a force to actuate the piston, and a cover member for closing the open end of the cylinder. The cover member, in association with the cylinder and the piston, defines an oil chamber in the cylinder. The oil pressure to the oil chamber is usually fed from an oil passage which is opened in the circumferential wall of the cylinder. A typical example of this hydraulic servo device is disclosed in U.S. Pat. No. 4,787,494.
As shown in FIG. 4, if the oil pressure is fed to the oil chamber from the oil passage opened in the circumferential wall of the cylinder, a portion for feeding the oil pressure has to be retained between a piston p and a cover member r. In addition, seal members S.sub.P and S.sub.R of the O-rings have to be placed around the entire circumference of the opening to prevent oil leakage. Therefore, even at the return stroke end of the piston p, as shown at the upper half of the figure, a predetermined axial clearance D has to be kept between the seal member S.sub.P of the piston p and the seal member S.sub.R of the cover member r. Because the two seal members S.sub.P and S.sub.R are full circumference seals, this arrangement implies that the piston seal S.sub.P, an oil passage opening q, and the cover member seal S.sub.R are arranged in series in the axial direction of a cylinder c. As a result, the axial length L of the cylinder c is the sum of the approximate length for the required piston stroke for the cylinder, and the length for forming the oil passage opening q.
However, as shown above, the hydraulic servo device of the band brake is arranged to extend radially inward from the transmission case to the speed change mechanism. Therefore, since the hydraulic servo device has a large axial length, the gear train of the speed change mechanism and the shape and arrangement of the related members cause the size of the entire mechanism of the automatic transmission to be enlarged. Thus, the large size of the resulting automatic transmission makes it difficult to mount on a vehicle.